Electronic control of fuel injection to combustion engines in vehicles is used today due to the advantages enabled through the electronic control in comparison with a mechanical control system. Electronic control has significantly contributed to make e.g. the diesel engine more powerful, more efficient, cleaner and quieter. U.S. Pat. No. 5,131,371-A discloses a part of such an electronic control system for a diesel engine.
A demanded fuel supply to a diesel engine is typically substantially proportional to a requested engine torque. The actual fuel supplied to the cylinders of the engine shall ideally be directly proportional to the demanded fuel supply. Hence the actual fuel supply shall ideally be proportional to the requested engine torque. However, in some fuel injection systems the correspondence between the actual fuel supply and the demanded fuel supply has not been satisfying during certain work conditions and demanded fuel quantities. This unsatisfying correspondence is caused by hydraulic instability in the fuel injection system, where the hydraulic instability may be caused e.g. when a fuel quantity control valve in the fuel injection system closes a fuel passage in the valve. The valve may for instance comprise a closing means, which is forced against a stop surface when closing the passage. The passage needs to be closed rapidly and the closing means may therefore bounce on the stop surface when closing the passage, thus enabling undesired leakage of fuel through the passage. This leakage causes the actual fuel quantity to differ from the demanded fuel quantity. If the demanded fuel quantity is small, the leakage is relatively high compared to the demanded fuel quantity. This makes the fuel injection control more difficult. A driver of the vehicle experiences the hydraulic instability through e.g. undesirable and distracting noise.
In some systems, hydraulic instability causes problems when a small increase of fuel supply is demanded and the current fuel supply is relatively low. The hydraulic instability here causes a decrease of torque and actual fuel supply although a higher fuel supply than in the injection cycle before is demanded (see also FIG. 1). To avoid the danger of an unstable fuel injection control system caused by this non-linear correspondence between the demanded fuel supply and the actual fuel supply, the fuel injection control system must be more stability robust than it would have to be if the non-linearity would not exist. There also has to be higher demands on the insensitivity of the fuel injection control system in order to keep it sufficiently accurate and reject disturbances. A way to compensate for the non-linearity is to develop a compensation routine for the control system, but this adds to the complexity and the computing time and is not accurate since the range of non-linearity depends on the individual vehicle configuration and the temperature of the fuel.
In order to avoid the hydraulic instability problems during the most frequently used driving conditions, fuel injection systems usually are designed in such a way that the hydraulic instability affects the fuel injection system within a range of low engine torque values. The fuel supply in this range is preferably designed to be lower than the fuel supply during idle speed. Hydraulic instability is however likely to affect the fuel supply also in ranges above idle speed. There are driving conditions wherein the fuel supply may be within the ranges where instability occurs, such as during cruise control at relatively low engine torque and during electronically controlled automatic or semi-automatic gear shifting in a smooth way.